When you harvest berries, it has a chance to drop those seeds. Some berries dont actually require x2 very seeds, it can be the lower version ( plain seed) instead. Also depending on the berry, they will drop one certain seed. So like figy berries, it will drop very and a rawst would be plain.(Figy is very spicy and you can make those with plain seeds, so yes you could start with plain seed berries then moving up to very seeds). You could also buy seeds but not necessary. If you want seeds, I got some, I dont play this often anymore.